Oak Wilt Disease
Urban Forestry Consultants (Fox Chapel Borough’s Certified Arborist) have provided the information below regarding Oak Wilt Disease.
Oak wilt, sometimes dubbed the new Dutch elm disease, is one of the more serious diseases significantly affecting a major hardwood species in North America. In Pennsylvania, the losses from this disease occur mainly in oak forests along ridge tops in the south-central portion of the state and in Allegheny County. Over the past number of years, I know of confirmed cases in Fox Chapel Borough on W. Waldheim Road, Windsor Road, St James Place, Edgewood Road, the top of Hunt Road, numerous cases on Hickory Hill Road, Spring House Lane, Nantucket Drive, and Chapel Ridge Road.
All oak species are susceptible, but species in the red oak group, including black, pin, red, scarlet, and shingle oaks are very susceptible, while species in the white oak group, including burr, chestnut, post, swamp white, and white oaks, are fairly resistant. American, Chinese, and European chestnuts, and bush chinquapin are also susceptible.
The disease-causing fungus, Ceratocystis fagacearum, invades the vascular (water-conducting) system of the tree causing it to plug. This is very similar to the affect that Dutch elm disease has on elm trees. Symptoms may develop at any time of the season, but usually appear in late spring to early summer. Symptoms generally first appear at the top of the tree usually on one or more branches and then progress downward. The leaves wilt, turn dull or bronze and then fall. Premature shedding of foliage is a visible diagnostic symptom of oak wilt. Affected branches develop a brown discoloration in the outer layers of sapwood, just under the bark. On the ends of cut twigs, the discoloration may appear as a complete browning of one or more of the outermost annual rings. Species in the red oak group rarely survive more than a few months after infection. Species in the white oak group, however, often live for several years after infection and in some cases appear to have recovered.
The disease may be spread over short distances by three primary vectors: by natural root graft transmission of trees growing close together, via sap-feeding insects called “picnic beetles” (Nitidulae), and also by tree wounding insects such as oak bark beetles (Scolytidae). The insects contaminated with C. fagacearum spores introduce the fungus through existing wounds or wounds made through feeding. They spread the spores as they move short distances to feed on adjacent oaks. On average, oak wilt will move at a rate of 75 to 100 feet per year. It is thought that beetles are not responsible for long distance spreading of the fungus. Since the fungus remains viable under firmly attached bark, it is suspected that the transport of infected logs and firewood is one way the fungus can be moved long distances.
Oak wilt is controlled most effectively through a community program of early detection and prompt removal and handling of dying and dead trees.
1. Identify the problem. Obtain an accurate diagnosis that oak wilt is the cause of the wilting and defoliation.
2. Create a Buffer Zone. Where necessary, sever all root grafts to nearby oaks before removing the infected tree. Trenching: Two trenches are recommended. A narrow trench of 3 feet in depth should be dug between a healthy oak and an infected oak of the same species group, that are 50 feet or less apart. A secondary trench should be placed between the first healthy oak and second healthy oak. Cost of trenching is approximately $5 – $10 per linear foot. Additionally, the first healthy oak beyond the primary trench should be injected with a systemic fungicide (see no. 5). Where trenching is not an option, it may be necessary to remove additional healthy oaks in order to create an effective boundary. In extraordinary situations, a timbering operation may be considered. This would entail harvesting and marketing timber to offset conventional tree removal expenditures. The timbering process would require approval by the appropriate governing body.
3. Sanitation. Remove the infected tree(s). Cover, bury, debark, burn, or remove all wood and stumps from infected tree(s). The fungus will survive in roots for 1 to 4 years, even in a dead tree. Ideally, infected wood with firmly attached bark should not be transported because insects in it may leave and carry the fungus to other oaks. However, responsible tree companies will be aware of this and take the appropriate measures after the wood has been removed from the site. It is always good practice to ask your contractor how the wood will ultimately be disposed of. Wood left on site may be buried, debarked, burned, or covered with 4 to 6 mil clear plastic. The edges of the cover must be buried or sealed to the ground. to prevent insect spread. Wood should remain covered for at least one year. If it is impossible to remove all of the wood before the beetles emerge, the wood can be sprayed with a registered insecticide until disposal is possible.
4. Pruning. Do not prune oaks during periods of possible insect activity. Fresh wounds are attractive to insects that may carry the fungus. Ideally, pruning should only be done in the safe period from December 1 through February 15. However, research suggests that the insect activity begins in mid-April and starts to decline towards the end of July, possibly providing a longer pruning window. The following guidelines are suggested:
Safe Period: December 1 through February 15
Low Risk Period: February 15 through March 31 &
August 1 through November 30
High Risk Period: April 1 through July 31
If a tree must be pruned, or is wounded in some other way during critical periods of insect and fungus activity, wounds/cuts greater than ½ inch in diameter should be painted with a good quality tree paint. Additionally, all pruning tools used on trees suspected of infection should be sanitized before use on a healthy oak tree. This can be accomplished with a 10% mixture of household bleach and water or with Lysol spray.
5. Tree Injection with Systemic Fungicide. While, there are no absolute therapeutic (curative) measures recommended for a tree once it has become infected, there are 2 systemic fungicides registered for use as a preventative measure. Alamo (sodium methyl dithiocarbamate) from CibaGeigy Corp. and Tebuject (tebuconazole) from Mauget Corp. These fungicides must be injected into the tree every two years, or, in high disease risk areas, every year. The manufacturer of Alamo also claims to have some success when the product is used as a rescue treatment in infected oaks exhibiting less than 20% – 30% infection in the crown. This therapeutic treatment would only be recommended for a highly valuable specimen tree. Cost of tree injection is approximately $400 – $1,000 per tree depending on size.
NOTE: Any procedure involving application of any type of herbicide, fungicide, or insecticide must be performed by a qualified Arborist who is a Commercial Pesticide Applicator certified by the Pennsylvania Department of Agriculture.
See Urban Forestry Consultant’s website for more information and links regarding Oak Wilt Disease